The present invention relates to a screening deck for the screening of material, such as crushed stone, gravel or the like, that will herein be referred to as crushed stone, which expression is not intended to imply that the stone or gravel is of a particular size. The screening deck comprises screening elements through which the material falls.
In the mining and stone industries, it is in many cases important to fractionate (separate) crushed stone and gravel into fractions of different sizes. Ideally, each fraction would comprise particles of a prescribed size, but in practice each fraction typically includes particles that are somewhat larger or smaller than the prescribed size. Normally, the deviation from the prescribed size that is permitted according to industry standards is defined, e.g., 10 percent for oversized particles and 15 percent for undersized particles. It is, however, important that each fraction comprises a blend of particles within the permitted deviation range, since mixtures that deviate from the standard blends are prized lower.
In most cases, fractionating is done by supplying an unfractionated stream of crushed stone or gravel to a vibrating screen provided with screening elements including screening holes for allowing stones smaller than the screening holes to pass through the holes. The vibration pattern and the inclination of the vibrating screen are arranged so that the crushed stones continuously flow in one direction on the screen, ultimately exiting one side of the screen or falling through the holes in the screening elements.
In this way it is possible to fractionate the crushed stone stream into stones smaller than the screening holes and stones larger than the screening holes. For most applications, such a fractionating is not sufficient, since the resulting crushed stone fractions range in size from stone powder up to the screening hole size and from the screening hole size up to the largest stones entering the screen, respectively. One way of further fractionating the crushed stone into finer fractions is to run one fraction leaving the screen to a further screen, but a more common way of solving the problem is to use a screen with multiple screening decks on top of each other.
On a screen with multiple screening decks, the screening decks are provided with ever smaller screening holes the lower the deck is located. Due to gravity, stones smaller than the screening holes in an upper deck will fall down to the neighboring lower deck. Stones smaller than the screening holes in that deck will fall through the screening holes, either to a further lower deck or to a surface below the lowermost screening deck. Hence, as the crushed stones leave the screen, the fraction between two decks will contain stones ranging in size from larger than the hole size of the lower screening deck to smaller than the hole size of the upper screening deck.
A problem with screening decks is the wear which they undergo. As is well known by people skilled in the art, crushed stones are very abrasive, especially when they are vibrated in order to flow slowly over a screen. In order to reduce the wear, virtually all surfaces contacting the crushed stone can be clad with, or made of, rubber or polyurethane. The areas most exposed to wear are the edges of the screening holes. Hence, most screening decks are provided with exchangeable screening elements. This not only allows exchange due to worn elements, but also for exchange between screening elements of various screening hole sizes.
A system for exchanging screening elements in a vibrating screen for the screening of crushed rocks or gravel is described in SE-A-0 460 340 (corresponding to U.S. Pat. No. 5,085,324). The screen according to that invention includes a multitude of screening elements. The elements are at one end provided with snap locks for interaction with elongated stanchions provided on transverse carriers reaching across the screen. The other ends of the screening elements that are not provided with snap locks are jammed in place by means of an extension of a neighboring screen element.
One major problem with all screening decks is that the crushed stone material to be screened, i.e. stones or gravel, travel along a longitudinal path in the screening deck. The travel path of the material is also called the traveling direction. At the edges of the screening elements, there are no screening holes. Hence, the longitudinal connection area between two adjacent screening elements is not provided with holes. This means that if the material starts to travel close to the edges of the screening element, where no holes are placed, the material may travel over the entire length of the screening deck without encountering a screening hole. This problem is worsened by the fact that the screening elements are rectangular or square having symmetrically located holes, thus creating longitudinal paths without holes. One way of decreasing this problem has been to provide wedge-shaped obstacles on the screening element or on the edges of the screening elements that cause the material to change direction or at least move it transversely to the traveling direction.
Further, it is important that the material to be screened does not move so quickly and undistorted over the screening element that the material that should fall down through the holes has the possibility to pass over the holes.